The present invention generally relates to a water-absorbing agent and an organic EL device using the same. More particularly, the present invention relates to an improved technology making it possible to produce in large quantity a low-profile organic EL device with a high degree of reliability which is not affected by water or oxygen and maintains stable light emitting characteristics for a long time.
Generally, an organic EL device is a self-light emitting device having a light-emitting part of a laminate comprising an organic EL layer formed of a thin film containing a luminescent organic compound sandwiched between an anode and a cathode. A hole and an electron are injected into the thin film containing a luminescent organic compound and are re-combined to generate an exciton. When the exciton is deactivated, it emits fluorescence or phosphorescence.
Since the light-emitting part of the organic EL device is not resistant to water, the light-emitting part is hermetically sealed in a sealed enclosure of glass or metal so as not to be exposed to the air. Concretely, the light-emitting part is formed as a laminate on a glass substrate, covered with a sealing cap of glass or metal from the light-emitting part and laminated to be sealed, and is sealed in a sealed enclosure formed of the substrate and the sealing cap. A water-capturing agent, such as barium oxide (BaO), calcium oxide (CaO), and the like, is charged in the sealed enclosure for capturing water. As a result, water attached to the light-emitting part and water present in an atmosphere inside the sealed enclosure, as well as, water which permeates from the outside of the sealed enclosure to the inside of the sealed enclosure through a sealing faces can be captured.
FIG. 1 is a cross section showing an example of the structure of an organic EL device. As shown in FIG. 1, the anode 2 of a transparent conductive film of ITO, the organic layer 3 and the cathode 4 are laminated on the glass substrate 1 to form a light-emitting part, which is covered with the sealing cap 5 formed of metal, and the sealing cap 5 is bonded to the substrate 1 with the adhesive 6 to form a sealed enclosure. The anode 2 and the cathode 4 are adapted to penetrate hermetically through the sealing portion of the sealed enclosure to be lead outside and to drive the light-emitting part. The concave portion 7 is formed in the sealing cap 5. Powder BaO is charged in the concave portion 7 as a water-capturing agent 8 and covered with the water-permeable film 9. In this example, light-emission of the organic layer 3 transmits toward the lower position in FIG. 1 via the anode 2 and the glass substrate 1.
FIG. 2 is a cross section showing the another example of the structure of an organic EL device. In the example shown in FIG. 2, a sealing glass substrate 11 is used for the sealed cap 5 of metal used in the example shown in FIG. 1. A concave portion 10 is formed on one side of the sealing substrate 11 opposite the light-emitting part by a countersinking processing, such as sandblasting, etching, and the like. The water-capturing agent 12 formed by packing CaO powder with a water-permeable agent or a seat-like water-capturing agent is affixed on the inner surface of the concave portion 10 of the sealing glass substrate 11. Further, a smaller concave portion may be optionally formed in the concave portion 10, in which the water-capturing agent is placed. The remaining structures are the same as those shown in FIG. 1. In this example, light-emission of the organic layer 3 transmits toward the lower position in FIG. 2 via the anode 2 and the substrate 1.
FIG. 3 is a cross section showing a top-emission type organic EL device, in which a sealing plate-like glass substrate 13 is used instead of the sealing cap of metal 5 shown in FIG. 1 or the sealing glass substrate 11 shown in FIG. 2. The anode 2, the organic layer 3 and the light permeable cathode 14 are laminated on the glass substrate 1 to form a light-emitting part. Further, the inorganic water-barrier layer 15 is laminated on the light-emitting part. Further, the sealing plate-like glass substrate 13 is bonded to the inorganic water-barrier layer 15 via the adhesive layer 16. In this manner, a complete solid hermitical structure without space is formed between the substrate 1 and the sealing glass substrate 13. The UV rays-curable sealing agent 17, such as epoxy sealing agent, is placed on the peripheral portion of the adhesive layer 16 to be cured and seals the cured adhesive layer 16 between the substrate 1 and the sealing glass substrate 13. In this example, light-emission of the organic layer 3 transmits toward the upper position in FIG. 3 via the cathode 14, the adhesive layer 16 and the sealed substrate 13. The top-emission type organic EL device is disclosed in Japanese Laying-open Patent Publication No. 2002-231443.
In the organic EL device shown in FIGS. 1 and 2, the concave portion 7 is formed in the sealed cap 5 which seals the light-emitting part on the substrate 1 by a pressing, or the concave portion 10 is formed in the sealing glass substrate 11 by a countersinking process, and then the powder BaO or CaO, as a water-capturing agent, or the seat-like water-capturing agent using the same is placed in the concave portion 7 or 10. When the necessary amount of the powder-like water-capturing agent is charged in the concave portion 7 or 10, its thickness is about 0.2 mm at the minimum. Accordingly, the depth of the concave portion 7 or 10 must be 0.3˜0.5 mm. This creates such a problem that the thickness of the sealing substrate 11 or sealing cap 5 becomes large and the entire thickness of the organic EL device becomes large.
In the organic EL device using the metal sealing cap 5 in which the concave portion 7 is formed by such molding or the organic EL device using the sealing glass substrate 11 in which the concave portion 10 is formed by a countersinking process, and the size of the whole device is large, there has been problems that the strength of the device as a whole lowers, because the sealed enclosure is of hollow structure, and the sealed cap 5 or sealed substrate 11 is easy to be flexible and contact to the cathode 4 and the reliability as a light-emission device lowers.
In order to package a powder-like BaO or CaO as a water-capturing agent in a sealed enclosure, it is required to suppress surely the scattering of the powder. Therefore, there has been a problem that automation of assembly is hard.
In the organic EL device shown in FIG. 3, a TFT circuit for driving is formed on the surface of the device substrate 1 on the side of the anode 2, on which is formed a light-emitting part as a laminate. Therefore, the light-emission of the organic layer 3 of the light-emitting part can not be taken out of the anode 2 and is taken out of the cathode 14 via the sealing substrate 13 as described above. Since the powder-like BaO or CaO, as a water-capturing agent, or seat-like water-capturing agent using the same is not transparent, such a water-capturing agent can not be placed on the cathode 14 out of which is taken light-emission. Therefore, such an organic EL device has no water-capturing agent therein. Thus, the remaining water in the device or water permeating from the outside can not be captured, which makes it difficult to ensure the reliability.
The light-emitting part is protected from water by laminating the inorganic water-barrier layer 15 thereon. However, a very high cost is required for mounting the inorganic water-barrier layer 15. Moreover, the inorganic water-barrier layer 15 is in danger of forming a pinhole. Therefore, depending on only the inorganic water-barrier layer 15 as water-capturing measures is not suitable for measures to ensure the reliability for mass-production.